1. Field of the Invention
This invention relates to the detection of particles in a chamber in which a substrate is processed and, in particular, to the identification of the composition of such particles. Most particularly, the invention relates to the identification of the composition of particles in a semiconductor process chamber.
2. Related Art
A variety of devices (e.g., integrated circuit chips, flat panel displays, microminiature mechanical sensors) can be manufactured in whole or in part by appropriately processing a semiconductor substrate (e.g., a semiconductor wafer) in a semiconductor process chamber. Manufacturers of such devices are continually seeking to improve the yield of each of the semiconductor processes used in making the devices. An important aspect of improving the process yield of a semiconductor process is the reduction of the presence of contaminants in the semiconductor process chamber during processing of a semiconductor substrate. (This is true of the processing of other types of substrates in other types of process chambers, as well.) Identifying the source or sources of contaminants in a process chamber can be an important first step in reducing the presence of contaminants in the process chamber. To enable identification of the source(s) of contaminants in a process chamber, it is helpful to identify the composition of contaminants present in the process chamber. It is often desirable to be able to do this without opening the process chamber, since opening the chamber may result in an extended period of non-operation of the chamber. Further, it is desirable to be able to identify the composition of the contaminants with confidence, so that solutions to the contamination problem needn't be discovered by trial and error.
The presence of contaminants in a process chamber can be detected by examining a substrate that has been present in the chamber to determine whether particles are present on or within the substrate other than those that are expected to be present. This examination may also identify the composition of the detected contaminants; if not, further examination of the substrate can be performed to accomplish such identification. Herein, a substrate that is examined to detect and identify contaminants present on the substrate is referred to as a "test substrate." A test substrate can be either a substrate that has been processed in the chamber during the normal course of operation of the chamber (which is a substrate referred to herein as a "process substrate") or a substrate that has been inserted into the chamber--and, if desired, subjected to specified operation of the chamber--specifically so that the substrate can subsequently be examined to determine whether there are contaminants present on or within the substrate (herein referred to as a "proxy substrate"). Following, a method that has been used to enable the detection and identification of contaminants in a process chamber is described.
First, a test substrate is examined to determine if any defects (or potential defects) are present on the substrate. Herein, such examination of a substrate is termed "defect detection." A defect may be a contaminant, but may also be something else, such as a stain on the substrate or a void formed in the substrate. Defect detection on a semiconductor wafer (or other semiconductor substrate) can be accomplished, for example, using a wafer scanner. A wafer scanner optically scans a substrate and detects the presence of one or more anomalous optical sites. For example, on a patterned semiconductor wafer, one die on the wafer can be compared to another die, or, on an unpatterned wafer, one defined region on the wafer can be compared to another defined region. Or, a site on a substrate can be compared to the corresponding site on another substrate of the same type that has previously been scanned or to the corresponding site on an artificial image that represents a substrate of that type. An anomalous optical site found by the wafer scanner may, but need not necessarily, represent a defect (e.g., stain, void, contaminant); the confirmation of the existence of a defect may require performance of additional examination of the substrate, as described below. The location and/or size of each identified site is provided as output from the wafer scanner. A wafer scanner can be embodied by, for example, a laser scanning system or a conventional video camera system. Various types of wafer scanners are commercially available from companies such as KLA Instruments of San Jose, Calif. and Tencor Instruments of Milpitas, Calif. (which companies have recently been combined into a single entity).
If defect detection indicates that one or more defects may be present on the substrate being examined (e.g., if a wafer scanner detects the presence of one or more anomalous optical sites on the substrate), the substrate can be further examined to determine, first, whether an identified site is the location of a contaminant or some other defect (or, perhaps, does not really represent a defect), and, second, in the former case, the type (e.g., composition) of the contaminant at the identified site. Herein, such examination of a substrate is termed "defect analysis" and, as particularly applied to identify the type of a contaminant, "contaminant analysis." Defect analysis can be accomplished, for example, using a scanning electron microscope (SEM) system. In an SEM system, an SEM emits high energy electrons so that the electrons strike a defined region of a substrate, causing electrons ("secondary electrons") to be released from elements that are part of the substrate and/or material formed on or within the substrate in the defined region. The energy of a secondary electron corresponds to the element from which that secondary electron was released. The intensity of secondary electrons of different energies is measured and analyzed by a detector/analyzer of the SEM system, thereby enabling determination of the types of elements that are present in the defined region of the substrate (i.e., measurement of a large intensity of electrons of a particular energy indicates the presence of the element that corresponds to that electron energy). Various types of SEM systems for performing defect analysis are commercially available. For example, an SEM that can be used in an SEM system as described above is the 7500WS microscope made by JEOL of Peabody, Mass. A detector/analyzer that can be used in an SEM system as described above is the Voyager system made by Noran of Middleton, Wis. As indicated above and discussed in more detail below, contaminant analysis is an important aid in identifying the source of a contaminant, which information can be used, in turn, to enable reduction of the presence of contaminants in the process chamber, with consequent improvement in the process yield obtainable from the chamber.
It can sometimes occur that a contaminant has a composition that includes one or more elements that are the same as an element included in the composition of a test substrate. Thus, when defect analysis as described above is performed on the test substrate, it is often difficult, when the presence of an element that is included in the composition of the test substrate is identified, to determine whether the output of the defect analysis tool has resulted from the presence of the element in a contaminant or from the presence of the element in the test substrate. Such difficulty can arise for a number of reasons. For example, when the defect analysis tool is an SEM, if the energy of the high energy electrons that bombard the substrate is sufficiently high, secondary electrons from elements underneath the surface of the substrate (e.g., underneath a contaminant) can be caused to be emitted from the substrate and detected by the SEM. Additionally, as illustrated and described in more detail below with respect to FIGS. 5A and 5B, as the size of a particle being examined by a defect analysis tool decreases relative to the size of a field of view (i.e., an area of the substrate that is examined at a particular time) of the defect analysis tool, the measured intensity of secondary electrons emitted from an element that is part of the particle decreases relative to the measured intensity of secondary electrons emitted from elements that are part of the surrounding substrate material. In any event, the above-described method of defect analysis is typically inadequate to enable the confident identification of the presence of an element as part of a contaminant where the element is also part of the composition of the test substrate.
For example, as is well known, semiconductor devices are often formed from substrates (e.g., wafers) comprised primarily or entirely (excepting incidental impurities) of silicon (hereinafter, for convenience, such substrates will be referred to simply as silicon substrates). It is not uncommon for a process chamber used to process a silicon substrate to have silicon particles present therein that are contaminants. Previously, defect analysis used in identifying contaminants in such process chambers has been performed using a silicon substrate as a test substrate (either a process substrate or a proxy substrate). When the above-described defect analysis method indicates that the material at a particular location on or within a silicon test substrate is silicon, it has been impossible or, at least, very difficult to determine whether such silicon is part of the substrate, part of silicon or a composition including silicon that has been properly formed on or within the substrate, or part of a contaminant including silicon.
It can also occur that a contaminant has a composition that includes one or more elements that are the same as an element included in the composition of a material that has been intentionally formed on or within a test substrate (often, in this case, the test substrate is a process substrate). For reasons similar to those given above, defect analysis as described above may be inadequate to enable the confident identification of the presence of an element as part of a contaminant where the element is also part of the composition of material that has been intentionally formed on or within the test substrate, particularly when such material has been formed at or near the location of the region of the substrate being analyzed. Thus, for example, the identification of the presence on a semiconductor test substrate of oxygen or aluminum, both materials that are commonly formed on semiconductor substrates, may be difficult to pinpoint with confidence as part of a contaminant, rather than as material that is properly formed on the test substrate.
A further problem arises when an SEM (or similar tool) is used to perform defect analysis on a test substrate made of silicon. The number of secondary electrons released from a given amount of silicon is large relative to the number of secondary electrons released from the same amount of some other elements (e.g., carbon). Thus, a given amount of silicon will generally produce an SEM output signal (i.e., secondary electron intensity) that is correspondingly larger than that produced by the same amount of one of such other elements. If both silicon and one of such other elements are present in a region of a substrate which is being analyzed, the SEM output signal associated with the silicon may dwarf the SEM output signal produced by the other element --particularly when the SEM produces graphical output that is scaled to fit into a predefined area--so that an SEM output signal corresponding to such element that is indicative of the presence of that element in the region of the substrate being analyzed may not be perceived as such. In other words, the relatively strong SEM output signal produced by silicon may, aside from masking the presence of contaminants including silicon, also obscure the presence of contaminants containing other elements. Further, this problem is exacerbated as the size of a contaminant decreases relative to the field of view of the defect analysis tool, since the magnitude of the SEM output signal produced by the elements of the contaminant will decrease even more relative to the magnitude of the SEM output signal produced by the silicon of the test substrate.
Moreover, even if a contaminant includes elements that produce strong SEM output signals relative to the SEM output signal produced by silicon, if the contaminant is sufficiently small, the SEM output signal from the silicon may overshadow the SEM output signal produced by the elements of the contaminant anyway. In other words, the relatively strong SEM output signal produced by silicon tends to, in general, hide the presence of relatively small contaminants.
Notwithstanding the above-noted problems, silicon substrates have been used as test substrates, particularly for identifying contaminants in semiconductor process chambers, for a variety of reasons. For instance, silicon substrates are readily available, since they are the most commonly used type of substrate in manufacturing semiconductor devices. Along the same lines, if the test substrate is a process substrate, the test substrate will most likely be a silicon substrate, since, as indicated above, such substrates are commonly used to manufacture semiconductor devices. Additionally, silicon substrates are relatively inexpensive compared to other types of semiconductor substrates (e.g., gallium arsenide substrates). Also, a silicon substrate is usually compatible (e.g., does not produce any undesirable reactions or other effects) with the process chamber being evaluated (as is evident from the fact that a silicon substrate is typically used as a process substrate).